The present invention relates to a compensation circuit for a power amplifier and, in particular, to a compensation circuit for an output transistor in a flyback output circuit.
Power amplifiers designed to function as flyback output circuits typically operate as high current switching devices that drive inductive loads, such as a yoke mechanism of a horizontal deflection circuit of a display system employing a cathode ray tube. An output transistor in the flyback output circuit is coupled to receive an input current on its base electrode causing the transistor to conduct and to develop an induced storage charge in the base. A ramping saturation voltage is developed on the collector of the output transistor as current flow increases in the collector. A high, reverse current is applied to the base electrode to rapidly remove the storage charge on the base and turn the transistor off. Once the storage charge is removed from the base, the current flowing in the collector of the transistor drops to zero as the voltage on the collector spikes-up to a high value, for example 4 KV. The voltage spike immediately falls in the negative direction until it is clamped just below zero volts by damping diodes in the flyback circuit.
During the time period when the collector current in the output transistor is dropping to zero and the voltage is spiking up, power is being dissipated in the transistor. Attempting to reduce the power consumption of the transistor by reducing the saturation voltage level on its collector by increasing the current into its base only succeeds in increasing the storage time and decreasing the switching speed of the transistor which increases the power consumption. Decreasing the input current to the base of the transistor increases the saturation voltage level on the collector which causes sweep non-linearity and, again, power consumption. There exists an optimum base drive current that will produce the maximum efficiency and speed for the output transistor by minimizing the power consumption and storage time.
It is very difficult at the present time to maintain an optimum base drive in a fixed flyback output circuit. Temperature changes, variations in the collector load, known as pincushion correction, variations in the beta of the output transistor from device to device along with component variations in the flyback output circuit affect the output saturation of the transistor. What is needed is a variable base drive circuit for the output transistor that maintains the maximum speed and efficiency of the transistor with respect to component and environmental changes.